GPU's will be good for a while still. New GPUs can still be paid for in 4-8 months.

After that, it will depend on your utility prices and the BTC exchange rate.

Contrary to what was said earlier, there is (currently) a very good correlation between difficulty and BTC exchange rate. That means that, even though GPU's will mine a lot less BTC, the BTC will be worth more in fiat terms and this will probably offset the dip in BTC mined (so you can still pay your bills and keep your GPUs running).

This is of course an educated guess, as everything is in flux...